Since the availability of 3He is becoming scarce, it is crucial to develop new sensors, which are capable of detecting neutrons with high sensitivity. In addition, the potential dual mode of operation afforded by the simultaneous detection of gamma rays and neutrons with a single scintillator will dramatically reduce the complexity of the instruments, which will provide for their wide deployment. In order to achieve a low false detection alarm rate, the main detector requirements include high efficiency, good energy resolution, and the ability to differentiate between gamma and neutron events. While a number of new materials have been identified and are being developed at present, they are often expensive and not readily available. Efforts need to be undertaken to find better and less expensive materials for γ-ray and neutron detection.
Radiation Monitoring Devices, Inc. (“RMD”) has invented a number of scintillator materials with improved performance over classical materials. Some of these materials offer thermal and fast neutron detection simultaneously with gamma ray detection, providing multimode operation. One of the first such materials is Cs2LiYCl6 (CLYC), which offers (1) improved energy resolution of better than 4% at 662 keV; (2) efficient thermal neutron detection (2×higher than 3He at 10 atmospheres); and (3) excellent separation between gamma and neutron particles (>10−6). CLYC is already in a stage of intensive commercialization with 1″ and 2″ inch crystals becoming standard products. FIG. 1A shows 1″ and 2″ CLYC packaged detectors produced and offered by RMD. Detection instruments and systems based on CLYC are being developed internally and by a number of companies and national labs. As an example, in FIG. 1B is shown the Thermo-Scientific's commercial gamma-neutron pager based on CLYC. CLYC is supplied by RMD and its sister organization (Hilger Crystals).
FIG. 2 illustrates gamma and neutron capabilities of the CLYC material. It shows energy spectra measured with a CLYC crystal under moderated Am/Be excitation. The peak at the right side of the spectrum is due to thermal neutrons. For energy calibration purposes the graph also shows a 137Cs spectrum (red curve). Based on the position of the 662 keV peak, the neutron peak appears at ˜3.2 MeV gamma equivalent energy (GEE).
These new scintillation materials have exciting properties, but they are associated with several issues which currently increase the cost of their mass deployment. First issue is the production yield. RMD demonstrated that high quality elpasolite scintillators can be grown and 1″ and 2″ CLYC crystals are routinely produced and delivered. However, the yield is limited due to cracking during the cooling of the ingots, defects, inclusions, and secondary phases at the both ends of the ingot. All these result in higher cost for large size scintillators. While obtaining crystals with sizes above 2″×2″ is possible, currently the cost of such crystals might become prohibitive to build low cost instruments.
An additional issue with these new materials is that they are highly hygroscopic, which complicates their handling. The standard procedure is to encapsulate them in a metal enclosure with an optical window. While this process is well established and used in other “classical” scintillators like NaI:Tl or LaBr3:Ce, it introduces additional light loss due to the difference in the refractive index of the crystal, window and optical readout component. The encapsulation is done in dry atmosphere, but the sealing could become compromised during the years in operation, which could lead to a degradation of the performance and long term issues.
Recently RMD investigated novel plastic scintillators based on styrene and vinyltoluene monomers doped with wavelength shifters such as PPO for use in applications where neutron/gamma pulse shape discrimination (PSD) is required. Measurements show that the plastic scintillators fabricated at RMD have excellent optical quality, a good light yield (similar to commercial plastics such as BC-404 and BC-408), and excellent gamma-neutron PSD with a Figure-of-Merit (FOM) of >3 at 2.5 MeVee gamma equivalent energy threshold. A photograph of a selection of RMD's neutron plastic scintillators is shown in FIG. 3.
The plastic scintillators do not exhibit the same issues like the new inorganic scintillators. They have very low production cost, based on low cost components and very high, practically 100%, production yield. They can be produced in large volumes and realized in practically any desirable shape. Moreover, they are not hygroscopic, which significantly simplifies their handling and reduces the cost of their utilization.
The plastic scintillators have excellent properties, but they exhibit a number of performance limitations. First, they have low detection efficiency due to their low density close to 1 g/cm3. Second, they do not have photopeak efficiency for energies above 30 keV due to low-Z constituents.